1. Field of the Invention
The present invention relates to an optical recording medium in which a recording layer is optically changed by irradiation with laser light, thereby recording data onto the recording layer, and a method for manufacturing such an optical recording medium.
2. Description of the Related Art
Optical recording media such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) are widely used as information recording media. In recent years, an optical recording medium has attracted attention that enables recording of a larger amount of information with higher density by using blue or blue-violet laser light as irradiation light. Moreover, recording capacity of the recording medium can be increased by employing multilayer recording in which a plurality of recording layers are formed with light-transmitting spacers interposed therebetween.
Optical recording media are roughly classified into a ROM (Read Only Memory) type in which writing and rewriting of data cannot be performed, an R (Recordable) type which allows data to be recorded only once, and an RW (Rewritable) type in which data can be rewritten. A recording layer of the R-type optical recording medium should have optical characteristics that can be changed by irradiation with laser light. Moreover, it is important that the recording layer is durable or hardly degrades during long-term storage. As a material for the recording layer of the R-type optical recording medium, an organic dye was widely used conventionally. The organic dye conventionally used was a material that hardly absorbed ultraviolet rays and visible light rays having short wavelengths such as blue rays, and blue-violet rays that could accelerate chemical reaction. This properties contributed to suppression of the degradation of the recording layer.
However, the conventionally used organic dye hardly absorbed the visible light rays having short wavelengths such as blue and blue-violet rays. Thus, in case of using blue or blue-violet laser light as irradiation light, satisfactory change of optical characteristics could not be obtained and therefore recording of data could not be performed. Moreover, it was difficult to develop an organic dye that could provide satisfactory change of optical characteristics even in case of using blue or blue-violet laser light as irradiation light and hardly degraded during long-term storage.
On the other hand, an R-type optical recording medium is known that uses an inorganic material such as an oxide of Bi, Mo, Cu, Ag, or Pd, for a material of a recording layer (see Japanese Patent Laid-Open Publications Nos. 2003-48375 and 10-334507, for example). A part of the above inorganic materials can provide satisfactory change of optical characteristics even in case of using blue or blue-violet laser light as irradiation light, and hardly degrades by ultraviolet rays and visible light rays having short wavelengths such as blue or blue-violet rays that can accelerate chemical reaction. However, even when the above inorganic materials were used, the recording layer degraded by moisture contained in a substrate or moisture entering from the outside through the substrate or a covering layer in some cases.
The degradation of the recording layer can be suppressed by forming a dielectric layer or the like (through which moisture hardly permeates) on each side of the recording layer.
However, forming of the dielectric layer or the like makes the structure of the optical recording medium more complicated and increases the number of steps for manufacturing the optical recording medium. This results in increase of the cost of the optical recording medium. Moreover, in case of forming the recording layer of the inorganic material or forming the dielectric layer or the like, an expensive vapor deposition apparatus such as a sputtering apparatus has to be used. However, in order to form the recording layer or the dielectric layer or the like from different materials, it is necessary to use a plurality of vapor deposition apparatuses in accordance with the number of types of the materials. As a result, the cost of the optical recording medium also increases largely.